1. Field of the Invention
This invention pertains to the presentation of projected images by a projection means which is scanned in at least two directions, typically a cathode-ray tube which is scanned horizontally and vertically.
2. Description of the Prior Art
Conventional television and oscilloscope technology employs fixed deflection means such as permanent magnets or direct current superimposed upon varying deflection currents, in the same winding of a deflection yoke or applied in a separate winding, to determine the central location of a beam in the absence of deflection. Alternatively, although less commonly, the central location may be determined by a fixed potential applied to electric deflection plates, either independently to separate deflection plates, or as a constant component to which varying scanning potential is applied additionally.
Varying deflection to produce scanning is provided by appropriate time-varying currents in windings on deflection yokes, or by appropriate time-varying potentials between deflection plates. The location of the picture is conventionally determined in all cases by adjustment of the fixed deflection means, and its size is conventionally determined by adjustment of the amplitude of the scanning currents or potentials. In all cases known to the applicant, constancy of location and size is determined by means to preserve the constancy of such potentials or currents at their desired values. If a projection system is used, the relation of the picture produced on the cathode-ray tube screen to its location on the projection screen is determined by the mechanically determined mutual geometric relation of the two screens and any projection optics.
When the object of projection is merely entertainment, or communication of information not directly dependent upon the accuracy of the mapping of the image upon the projection screen, such means have been found satisfactory. But the scale of a representation upon a projection (or a directly viewed) system will clearly vary with variation in picture size; for precision presentation, such variation is objectionable. Also, since the deflection of a cathode-ray beam is an angular deflection, a shift in the position of the raster may cause a shift in the relative position of the elements of the raster with respect to each other, except in the unique case where the tube face is spherical and centered with respect to a virtual center around which the beam is deflected and (in a projection system) where the optical system is such that a shift in the raster will not alter the relative position of the elements of the raster in the projected image. This stringent requirement may be relaxed by controlling the position of the raster. When pluralities of projection screens are placed contiguously to afford a simulation of a continuous picture it is clearly necessary that pictures remain as nearly fixed in position as possible to preserve the required continuity.
While it is possible to achieve high degrees of constancy in electrical deflection systems, this is always done at a cost of high quality components which remain constant with temperature variations and age, and of increased complexity of circuitry. This approach has the inherent limitation that it is essentially a predictive or, in servo terms, open-loop system; it has no inherent capacity to correct for any unforeseen variation in the system whose object is to produce a correctly located picture of proper magnitude at the projection screen.